Generally, in a reciprocating compressor, a compression space to/from which an operation gas is sucked and discharged is defined between a piston and cylinder, so that the piston is linearly reciprocated inside the cylinder to compress refrigerant.
Since the reciprocating compressor includes a component for converting a rotation force of a driving motor into a linear reciprocation force of the piston, such as a crank shaft, a large mechanical loss occurs due to the motion conversion. Recently, a linear compressor has been actively developed to solve the foregoing problem.
In the linear compressor, particularly, a piston is connected directly to a linearly-reciprocated linear motor to prevent the mechanical loss by the motion conversion, improve the compression efficiency and simplify the configuration. Power inputted to the linear motor can be regulated to control the operation thereof. Accordingly, since the linear compressor can reduce noise more than the other compressors, it has been mostly applied to electric home appliances used indoors, such as a refrigerator.
FIG. 1 is a view illustrating an example of a conventional linear compressor.
In the conventional linear compressor, a structure composed of a frame 1, a cylinder 2, a piston 3, a suction valve 4, a discharge valve assembly 5, a linear motor 6, a motor cover 7, a supporter 8, a rear cover 9, main springs S1 and S2, a muffler assembly 10 and an oil supply device 20 is installed to be elastically supported inside a shell (not shown).
The cylinder 2 is fixedly fitted into the frame 1, the discharge valve assembly 5 composed of a discharge valve 5a, a discharge cap 5b and a discharge valve spring 5c is installed to block one end of the cylinder 2, the piston 3 is inserted into the cylinder 2, and the thin suction valve 4 is installed to open and close an outlet 3a of the piston 2.
In the linear motor 6, a permanent magnet 6c is installed to be linearly reciprocated, maintaining a gap between an inner stator 6a and an outer stator 6b. The permanent magnet 6c is connected to the piston 3 by a connection member 6d, and linearly reciprocated due to a mutual electromagnetic force between the inner stator 6a, the outer stator 6b and the permanent magnet 6c to thereby operate the piston 3.
The motor cover 7 supports the outer stator 6b in an axis direction to fix the outer stator 6b, and is bolt-fixed to the frame 1. The rear cover 9 is coupled to the motor cover 7. The supporter 8 connected to the other end of the piston 3 is installed between the motor cover 7 and the rear cover 9 to be elastically supported by the main springs S1 and S2 in an axis direction. The muffler assembly 10 for sucking refrigerant is fastened together with the supporter 8.
Here, the main springs S1 and S2 include four front springs S1 and four rear springs S2 in up-down and left-right positions symmetric around the supporter 8. When the linear motor 6 is operated, the front springs S1 and the rear springs S2 are driven in the opposite directions to buff the piston 3 and the supporter 8. Besides, refrigerant in a compression space P serves as a kind of gas spring to buff the piston 3 and the supporter 8.
The oil supply device 20 is composed of an oil supply tube 21, an oil pumping unit 22 and an oil valve assembly 23, and installed to communicate with an oil circulation passage (not shown) formed in the frame 1.
Therefore, when the linear motor 6 is operated, the piston 3 and the muffler assembly 10 connected thereto are linearly reciprocated. Since a pressure inside the compression space P is varied, the operations of the suction valve 4 and the discharge valve assembly 5 are automatically controlled. During the operation, refrigerant flows through a suction tube on the shell side, an opening portion of the rear cover 9, the muffler assembly 10 and an inlet 3a of the piston 3, is sucked into and compressed in the compression space P, and is externally discharged through the discharge cap 5b, a loop pipe and a discharge tube on the shell side.
Here, when vibration occurring due to the linear reciprocation of the piston 3 is transferred to the oil pumping unit 22, a pressure difference is generated by the oil pumping unit 22. Oil filled in the bottom of the shell is pumped through the oil supply tube 21 due to the pressure difference. The oil flows through the oil valve assembly 23, circulates along the oil circulation passage (not shown), and returns to the bottom of the shell. Such circulated oil serves to lubricate and cool components such as the cylinder 2 and the piston 3.
FIGS. 2 and 3 are views illustrating an example of the frame and the cylinder of the conventional linear compressor. The conventional frame 1 and cylinder 2 are insert-die-casted. In a state where the cylinder 2 is casted and inserted into a mold, the frame 1 is casted with Al. Here, the cylinder 2 is coupled to the center of the frame 1. A pair of holes 1a are formed in the frame 1 at both sides of the cylinder 2 to reduce an air resistance. An electric wire fetching groove 1b is provided to be open at one side of the frame 1 so that an electric wire connected to the linear motor 6 (refer to FIG. 1) can pass therethrough. Spring supporting portions 1c for supporting springs (not shown) for elastically supporting the structure are formed to protrude from both side lower portions of the frame 1. Besides, the oil circulation passage (not shown) for supplying oil to between the cylinder 2 and the piston 3 is defined in the frame 1. The oil supply tube 21 and the oil pumping unit 22 can be integrally formed with a lower portion of the frame 1, communicating with the oil circulation passage. The oil valve assembly 23 (refer to FIG. 1) can be individually bolt-fastened to the frame 1.
FIG. 4 is a graph showing fastening deformations of the frame and the cylinder of the conventional linear compressor. Referring to FIGS. 2 to 4, in a state where the frame 1 and the cylinder 2 are insert-die-casted, when radius direction distances from the center of the cylinder 2 are 5.65, 10, 63 and 68 mm, fastening deformations of the frame 1 and the cylinder 2 are shown. The more the radius direction distance from the center of the cylinder 2 increases, the more the fastening deformation of the frame 1 and the cylinder 2 increases in specific directions, i.e., directions of the holes 1a and the electric wire fetching groove 1b. 
Accordingly, in the conventional linear compressor, when the size of the frame 1 is limited and the size of the cylinder 2 is increased, since the holes 1a are formed in portions of the frame 1 adjacent to the installation portion of the cylinder 2, structurally, a fastening portion 1 in of the frame 1 brought into contact with the cylinder 2 is too thin in consideration of the size of the cylinder 2. As a result, the strength of the frame 1 is reduced, so that the deformation of the frame 1 is transferred to the cylinder 2, causing a large fastening deformation thereto. When the piston 3 (refer to FIG. 1) is linearly reciprocated, the piston 3 (refer to FIG. 1) is brought into contact with the deformed cylinder 2, which results in low operation reliability.